Eleven of the macaques in these studies were part of a vaccine evaluation experiment. Six had been vaccinated previously with vaccinia virus expressing HIV-1LAI gp160 and subsequently boosted with purified subunit gp160 produced by vaccinia virus-infected AGMK cells; the remaining five are mock-vaccinated controls that received vaccinia virus alone and boosted with IFA. Pre-virus challenge evaluation of the animals revealed serological responses similar to those achieved in humans immunized with various env preparations. The macaques were challenged with 25 tissue culture infectious doses (TCID50). All 6 mock vaccinated animals became persisently viremic by virus culture and RNA PCR. In contrast, virus was recovered intermittently from just 1 of 6 vaccinated animals and the RNA PCR revealed virus levels several logs lower in the vaccinated animals. The animals will continue to be monitored and we plan to rechallenge with a more virulent SHIV. In a second challenge experiment , 14 M. nemestrina remained (from the original 22), which had been divided among experimental vaccine and control groups, also challenged with SHIVIIIB (SHIVHXBc2). Results from this SHIV-challenge experiment show that vaccinia env prime and env boost afforded the greatest protection from challenge. The animals will continue to be monitored for progression to disease. Two macaques from the above control groups experienced spontaneous CD4 decline; whole blood was collected for in vivo passage. The recipient macaques experienced a rapid and irreversible CD4 decline. Virus was recovered from the transfusion recipients and stocks were prepared. After the pathogenicity of the stock was confirmed in vivo, an in vivo titration was initiated to determine the minimal infectious dose. FUNDING NIH grant RR00166.